The present invention relates generally to feature measurement in scanning electron microscopy, and more specifically to apparatus and methods for enhancing image quality. The present invention may also be applied to feature measurement and image enhancement in similar instruments.
FIG. 1 is a diagrammatic representation of a conventional scanning electron microscopy configuration 100. As shown, a beam of electrons 102 is scanned over a specimen 104 (e.g., a semiconductor wafer). Multiple raster scans 112 are typically performed over a small area 114 of the specimen 104. The beam of electrons 102 either interact with the specimen and cause an emission of secondary electrons 106 or bounce off the specimen as backscattered electrons 106. The secondary electrons and/or backscattered electrons 106 are then detected by a detector 108 that is coupled with a computer system 110. The computer system 110 generates an image that is stored and/or displayed on the computer system 110.
Although conventional microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the specimen for some applications. For example, on a specimen made of a substantially insulative material (e.g., a semiconductor material), performing multiple scans over a small area sometimes causes the specimen to accumulate excess positive charge in the small area relative to the rest of the specimen. The excess charge generates a potential barrier for some of the secondary electrons, and this potential barrier inhibits some of the secondary electrons from reaching the detector 108. Since this excess charge is likely to cause a significantly smaller amount of secondary electrons to reach the detector, an image of the small area is likely to appear dark, thus obscuring image features within that small area.
Additionally, conventional systems and techniques fail to produce images for certain specimen features. For example, the bottom of a contact or trench region is typically undifferentiated from the adjacent sidewalls of the contact or trench. Typically, both the bottom and sidewalls will appear dark because a significant number of the secondary electrons within the contact or trench hit the sidewalls and fail to escape from the contact or trench and reach the detector 108. As a result of this failure, the bottom and sidewall""s individual dimensions and shapes are obscured within the resulting image.
Thus, microscopy apparatus and techniques for improving image quality are needed. More specifically, mechanisms for controlling charge distribution on the surface of the specimen are needed.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for controlling charge distribution on the specimen so as to improve image quality. Charge is controlled by multiplexing an image scanning phase and a setup scanning phase. The image scanning phase may be used to generate an image from the specimen, and the setup phase may be used to control charge that may have resulted from the image scanning phase. While the image phase has operating conditions that are selected to generate an image, the setup phase has operating conditions selected to control charge. Thus, the setup scanning phase has different operating conditions than the image phase such that charge is controlled during the setup phase. The present invention implements multiplexing of one or more values for one or more of the following operating conditions: scanning area, landing energy, beam current density, and scan pattern.
In one embodiment, a method of generating an image of a specimen with a scanning electron microscope (SEM) is disclosed. The SEM has a source unit for directing an electron beam substantially towards a portion of the specimen, a detector for detecting particles that are emitted from the specimen, and an image generator for generating the image of the specimen from the emitted particles. The image features are controlled by conditions under which the image is generated.
The specimen is scanned under a first set of conditions to generate a first image during a first image phase. The specimen is then scanned under a second set of conditions during a setup phase. The second set of conditions are selected to control charge on the specimen. The specimen is then scanned under the first set of conditions to generate a second image during a second image phase. The features of the second image are controlled by the first and second sets of conditions.
In a specific embodiment, the first set of conditions includes a first landing energy of the electron beam, the second set of conditions includes a second landing energy, and the second landing energy used during the set up phase is selected to improve the second image by increasing or decreasing charge that is built up on the specimen during the first image phase. In one implementation of the invention, the first landing energy and second landing energy are controlled by biasing a support that is positioned adjacent to the specimen.
In another specific embodiment, the first set of conditions includes a first current density of the electron beam, the second set of conditions includes a second current density, and the second current density used during the set up phase is selected to improve the second image by increasing or decreasing a rate at which the second image phase may be conducted. In yet another embodiment, the first set of conditions includes a first scan pattern of the electron beam over the specimen, the second set of conditions includes a second scan pattern over the specimen, and the second scan pattern is selected to improve an image quality parameter (e.g., resolution, contrast, signal to noise ration, and/or topology information content) of the second image by controlling charge distribution on the specimen. For example, a scan pattern may be a circular pattern as opposed to a raster scan pattern.
In an apparatus aspect of the invention, a scanning electron microscope (SEM) for generating an image from a specimen is disclosed. The SEM includes an electron beam generator arranged to generate and control an electron beam that is directed substantially towards the specimen and to receive an image control signal that indicates a parameter setting of the electron beam. The SEM also includes a detector arranged to detect charged particles emitted from the specimen to allow generation of an image from the detected charged particles.
The SEM further includes a multiplexer control system arranged to receive a first input signal, a second input signal, and a phase control signal that indicates an image phase or a setup phase. The multiplexer control system is further arranged to output the image control signal to the electron beam generator. The image control signal is based on the first input signal when the phase control signal indicates the image phase, and the image control signal is based on the second input signal when the phase control signal indicates the setup phase.
In another aspect, a method of generating an image of a high-aspect ratio feature of a specimen using a scanning electron microscope (SEM) is disclosed. The SEM has an electron beam generator for generating an electron beam that is configurable to direct the electron beam across the specimen in various scan patterns. The feature has a first wall portion that has a position that is not aligned with a second wall portion.
A first electron beam is scanned in a pattern on the specimen to generate a charge distribution on the specimen proximate the feature such that charge is controlled on the first wall portion so as to control a trajectory of emitted particles near the first wall portion. A second electron beam is scanned across the feature to generate an image of the feature, wherein the charge distribution bends the trajectories of secondary electrons generated during the scanning out of the high-aspect ratio feature.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.